The present invention generally relates to liquid crystal display devices and more particularly to the driving of an active-matrix liquid crystal display device in which representation of images is achieved by applying a driving voltage to a liquid crystal layer via a thin-film transistor (TFT).
Liquid crystal display devices have various advantageous features such as compact size, light weight, low power consumption, and the like. Thus, liquid crystal display devices are used extensively in portable information processing apparatuses such as lap-top computers or palm-top computers. Further, liquid crystal display devises are used also in desktop computers in these days.
A typical liquid crystal display device includes a liquid crystal layer confined between a pair of glass substrates and achieves representation of images by inducing a change in the orientation of liquid crystal molecules in the liquid crystal layer by applying a driving voltage to the liquid crystal layer. Such a change in the orientation of the liquid crystal molecules causes a change in the optical property of the liquid crystal layer.
In the case of using such a liquid crystal display device in a high-resolution color representation apparatus, there is a need of driving the individual pixels or liquid crystal cells defined in the liquid crystal layer at a high speed. In order to meet this requirement, it is generally practiced to provide a thin-film transistor in correspondence to each of the pixels in the liquid crystal layer and to drive the liquid crystal cells by way of such thin-film transistors.
FIG. 1 shows the construction of a liquid crystal panel 10 used in such an active matrix liquid crystal display device of a related art in a plan view, while FIG. 2 shows the part circled in FIG. 1 in a cross-sectional view.
Referring to FIG. 2, the liquid crystal panel 10 generally includes a pair of glass substrates 10A and 10B, and a liquid crystal layer 10C is confined between the substrates 10A and 10B.
As represented in the plan view of FIG. 1, the glass substrate 10A carries thereon a number of thin-film transistors 111-114 corresponding to the pixels in a row and column formation, wherein the thin-film transistors 111 and 112 aligned in the row direction are connected commonly to a gate bus line G1 provided directly on the glass substrate 10A. Similarly, the thin-film transistors 113 and 114 are connected commonly to a gate bus line G2 provided directly on the glass substrate 10A. Further, the glass substrate 10A carries thereon a number of generally H-shaped auxiliary electrodes Cs at the level of the gate bus lines G1 and G2, wherein the auxiliary electrode Cs is covered by an insulation film 12 as represented in the cross-sectional view of FIG. 2, and data bus lines D1 and D2 are formed on the insulation film 12 so as to extend in the column direction as represented in the plan view of FIG. 1.
It should be noted that the data bus lines D1 and D2 are covered by another insulation film 13 as represented in the cross-sectional view of FIG. 2, and the data bus line D1 is connected to the respective source regions of the thin-film transistors 111 and 112 via a conductor pattern branched from the data bus line D1. Similarly, the data bus line D2 is connected to the respective source regions of the thin-film transistors 112 and 114 via a conductor pattern branched from the data bus line D2.
Further, there is provided a rectangular pixel electrode of a transparent conductor such as ITO on the insulation film 13 in correspondence to the drain region of each of the thin-film transistors. For example, the drain region of the thin-film transistor 111 is connected to a transparent pixel electrode P1 provided on the insulation film 13 via a contact hole formed in the insulation film 13. As can be seen from FIGS. 1 and 2, the auxiliary electrode Cs is disposed at both sides of the data bus line D1 or D2 when viewed in the direction perpendicular to the substrate 10A, such that the electrode Cs overlaps the edge part of the transparent pixel electrode P1 or P2. Thereby, the auxiliary electrode Cs forms an auxiliary capacitor together with the transparent pixel electrode P1 or P2.
Further, each of the transparent pixel electrodes P1 and P2 is covered by a molecular alignment film 14, wherein the molecular alignment film 14, contacting directly with the liquid crystal layer 10C, induces an alignment of the liquid crystal molecules in the liquid crystal layer 10C in a predetermined direction.
The opposing substrate 10B, on the other hand, carries a color filter CF in correspondence to the foregoing transparent pixel electrode P1 or P2, and a transparent opposing electrode 15 of ITO, and the like, is provided uniformly on the substrate 10B. It should be noted that the transparent opposing electrode 15 is covered by another molecular alignment film 16, and the molecular alignment film 16 induces an alignment of the liquid crystal molecules in the liquid crystal layer 10C in a desired direction. Further, the substrate 10B carries thereon an opaque mask BM in correspondence to a gap between a color filter CF and an adjacent color filter CF.
FIG. 3 shows an example of the driving signal supplied to the data bus line D1 or D2 when driving the liquid crystal panel 10 of FIGS. 1 and 2.
Referring to FIG. 3, a bipolar driving pulse signal is supplied to the data bus line from a driving circuit, wherein it should be noted that the bipolar driving pulse signal changes a polarity thereof between a positive peak level of +VD and a negative peak level −VD during the black mode of the liquid crystal panel 10 for representing a black image. Further, a predetermined common voltage VCs is supplied to the opposing electrode 15 and the auxiliary electrode Cs from another D.C. voltage source during the black mode. In the white mode of the liquid crystal panel 10 for representing a white image, on the other hand, on the other hand, a bipolar drive pulse signal having an amplitude smaller than a predetermined threshold voltage is supplied to the foregoing data bus line D1 or D2.
It should be noted that the foregoing D.C. voltage source for supplying the common voltage VCs is provided as an independent unit independent from the driving circuit used for driving the data bus line D1 or D2. The D.C. voltage source provides a voltage of ΔVc as the foregoing common voltage VCs, wherein the common voltage VCs thus set is slightly offset from the central voltage Vc of the bipolar driving pulse signal. It should be noted that the liquid crystal panel 10 of FIG. 1 or 2 uses a low voltage liquid crystal, characterized by the black mode drive voltage VD of about 5 V or less, for the liquid crystal layer 10C.
In the liquid crystal panel 10 driven as such, it should be noted that the optimum common voltage VCs changes slightly between the black representation mode and the white representation mode. More specifically, the optimum common voltage VCs coincides substantially with the central voltage Vc of the bipolar driving pulse signal (ΔVc=0) in the black representation mode, while the optimum common voltage deviates from the central voltage Vc (ΔVc≠0) in the half-tone or white representation mode. As the common voltage VCs is applied uniformly to the opposing electrode 15, it is difficult to change the common voltage adaptively depending on the content of the image to be represented. Thus, it has been practiced to fix the common voltage VCs to the optimum voltage at the time of the half-tone representation mode.
Meanwhile, the inventor of the present invention has noticed, in a liquid crystal panel using a low voltage liquid crystal for the liquid crystal layer 10C, that there appears a noticeable flicker in the represented images along the edge part of the auxiliary electrode Cs. In the investigation that constitutes the foundation of the present invention, the inventor has studied this phenomenon and discovered that the flicker is caused as a result of variation of the disclination which is induced in the liquid crystal layer 10C in the region including the data bus line D1 or D2 and the auxiliary electrode Cs by a strong lateral electric field.
FIGS. 4A and 4B show the alignment of the liquid crystal molecules in the liquid crystal layer 10C and the electric flux of the lateral electric field applied to the liquid crystal layer for the case in which the common voltage VCs applied to the auxiliary electrode Cs and the opposing electrode 15 is offset from the central voltage of the bipolar driving pulse signal (VCs≠Vc, wherein FIG. 4A shows the state in which a voltage of +5V is applied to the data bus line D1 or D2 (represented as “D”), while FIG. 4B shows the state in which a voltage of −5V is applied to the data bus line D.
Referring to FIG. 4A, it can be seen that a very large lateral electric field is created between the data bus line D and the adjacent auxiliary electrode Cs in the state the voltage of +5V is applied to the data bus line D. Associated with this, there occurs a conspicuous disturbance in the molecular orientation or disclination in the liquid crystal layer 10C in correspondence to the part between the data bus line D and the auxiliary electrode Cs. As a result of the formation of such a disclination, there is induced a domain structure in the liquid crystal layer 10C, and a leakage of light occurs in correspondence to the boundary of the domains as represented in FIG. 4A by arrows.
In the state of FIG. 4B in which a voltage of −5V is applied to the data bus line D, on the other hand, the lateral electric field applied to the liquid crystal layer 10C is substantially reduced and there occurs no substantial formation of domain structure or associated problem of leakage of the light. As the state of FIG. 4A and FIG. 4B appears alternately in correspondence to the polarity of the bipolar driving signal pulse, the leakage light appearing only in the state of FIG. 4A causes the flicker.
Further, the inventor of the present invention has discovered that there occurs a flow of the liquid molecules in the liquid crystal layer 10C in the rubbing direction of the molecular alignment film when the value of the common voltage VCs of the auxiliary electrode Cs is deviated from the central voltage of the bipolar driving pulse signal. When such a flow occurs in the liquid crystal layer 10C, there occurs an increase in the thickness of the liquid crystal layer 10C in correspondence to the part where the liquid crystal molecules are accumulated. When there occurs such a change in the thickness of the liquid crystal layer 10C, the optical property of the liquid crystal panel 10 is modulated also.
Further, in the case a low-voltage liquid crystal is used for the liquid crystal layer 10C, there tends to occur a sticking of images as a result of the accumulation of impurity ions associated with the flow of the liquid crystal molecules. It should be noted that such a low-voltage liquid crystal, characterized by a low driving voltage, is particularly vulnerable to contamination.